Orthogonal frequency division multiplexing (OFDM) is a modulation scheme which divides a frequency band into a number of equally spaced frequency subcarriers (or tones) and data is then modulated onto these subcarriers. OFDM is used in wireless local area networks (WLAN), digital video broadcasting (DVB), digital audio broadcasting (DAB) etc., because of its high spectral efficiency, high data rate capability and resistance to multipath fading. However, OFDM suffers from a large peak-to-average-power-ratio (PAPR) problem: when the number of subcarriers is large, contributions from these subcarriers may coherently add together to form large peaks, which are well above the mean amplitude level.
The high PAPR value causes difficulties in power amplifier (PA) design. If the PA back-off is less than the PAPR value, nonlinearity of the PA (especially clipping) can cause in-band and out-of-band distortion. Out-of-band distortion causes spectral splatter which increases adjacent channel interference while in-band distortion reduces the modulation accuracy of the transmitted signal. On the other hand, if the PA back-off is higher than the PAPR value, PA power efficiency is reduced significantly.
Digital signal processing of the baseband OFDM signal can be used to reduce its PAPR value and a number of algorithms have been proposed which can be divided into three categories. Those PAPR reduction algorithms in the first category use redundant coding, i.e. they select codewords with low PAPR values for transmission by exploiting the redundancy of codewords; however this involves high computation complexity to search for suitable codewords and therefore the technique is more suitable for scenarios with small numbers of subcarriers (where the problem of PAPR is not serious). Furthermore, this is not suitable to implement communication standards that do not provide the required codeword redundancy. The second category of algorithms reduce PAPR by clipping the baseband signal amplitude from the output of the inverse fast Fourier transform (IFFT); however, this causes serious adjacent channel interference (ACI). Algorithms in a third category perform peak cancellation using unused subcarriers and either require multiple FFT/IFFT blocks or iterative processing with long latency and high complexity.
As described above, the existing algorithms for performing PAPR reduction either introduce serious interference and distortion or result in significant implementation complexity and processing latency. Many communication standards (e.g. the IEEE 802.11 standard) require transmissions to occur within a limited time after their payload is known to the transmitter, which makes algorithms with high complexity and long processing latency impractical for real-time implementation.
The embodiments described below are not limited to implementations which solve any or all of the disadvantages of known methods of peak reduction.